Charlie99
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It's not that you are getting "credit" from anything. The PADI RDP table (and most other tables too) simply track one (and only one) compartment. The PADI RDP tracks the 60 minute compartment, and the different pressure groups correspond to specific levels of loading in the 60 minute compartment. 10 minutes at 80' loads the 60 minute compartment to the same level as does 14 minutes at 60', as does any other depth/time combo that corresponds to pressure group C. That's why using zero SI and RNT to start the next multilevel segment works.Theoretical Dive: Wreck in 100'. Planned visits are the hull at 80', the deck at 60', and the bridge at 30'. For ease of calculations, plan for 10 minutes at each site. We also are assuming this is the first dive with no nitrogen buildup to begin with. Dive 1 is using a completely square profile, Dive 2 is using my 0:00 SI method. These use the RDP.
Dive 1: 80' for 30 min. = PG R (which is actually the NDL for 80'.)
Dive 2A: 80' for 10 min. = PG C
0:00 SI leaves 14 min. RNT at 60'
Dive 2B: 60' for 24 min. (14+10) = PG I
0:00 SI leaves 48 min. RNT at 35'
Dive 2C: 35' for 58 min. (48+10) = PG L
Credit is obviously being given for something, as at the end of the dives, there is a PG difference of 6. My question is whether this credit comes from somewhere we don't want it to or not. Can anybody with software/computer run this and see if it cries?
The zero SI method you used above provides you with a good estimate of the loading in the 60 minute compartment at the end of the series.
Although the RDP tracks ONLY the 60 minute compartment, the NDL is determined by calculating the loadings of all compartments. The faster compartments that control dives deeper than 40' will reach their limit faster than does the 60 minute compartment ---- that's why for example, that the 30 minute NDL of an 80' dive only gets you out to pressure group R.
While the zero SI method you described correctly tracks the 60 minute compartment, you can come up with multilievel profiles that stay within the limits of the RDP yet are slightly outside the NDL limits on the faster compartments. That's why the PADI wheel has slightly reduced NDLs for multilevel dives. I've put together a table that has the reduced NDLs (PADI Wheel calls them MDLs), but in practice I've found that just using the PADI RDP with the zero SI method is close enough to do a sanity check pre-dive on a proposed multilevel profile, and then I use my computer realtime as I dive.
Charlie Allen